The extraction and utilization of geothermal heat in existing heat plants, and in heat pumps, is well known. Existing geothermal heat plants extract heat from deep in the earth in the form of hot water or steam which has flowed through geological rock formations containing geothermal heat, i.e., natural hot water or steam present in the ground. This sometimes produces a very toxic condensate which poses serious environmental problems.
In contrast, with heat pumps it is an established practice to use heat exchangers with considerable surface area at a relatively shallow depth so that, in spite of a comparatively small temperature gradient, it is possible to obtain a reasonable degree of efficiency. Because of structural limitations in known heat pump installations, however, the amount of surface contact area available between the heat transmission medium and the earth's interior is relatively small; a factor which limits the degree of efficiency attainable with such plants.